The present invention relates generally to television signal transmission and receiving systems and, more particularly, to a system wherein the horizontal synchronization components of a conventional NTSC television signal are suppressed providing a scrambled transmission format particularly useful in subscription or pay-television applications.
In subscription television systems programming signals are typically transmitted, either "over-the-air" or through a suitable cable network, in a scrambed form rendering the broadcast video information largely unviewable when received by a conventional television receiver. In order to unscramble the video display, each system subscriber is provided with a decoder operable for unscrambling the broadcast signals and for coupling the unscrambled signals to a conventional television receiver for viewing. Security is, of course, a prime consideration in the design of such systems. That is, the scrambling technique employed should be of a nature making the unauthorized decoding or unscrambling of the broadcasts difficult in order to deter the would-be pirate. At the same time, the scrambling technique must accommodate reliable decoding by authorized system subscribers.
One technique commonly used for scrambling the video display produced in response to a broadcast television signal is that of horizontal sync suppression. Suppression of the horizontal synchronization components of a broadcast television signal below most video levels will cause the deflection circuits of a normal television receiver to behave eratically such that a scrambled video image is produced on the display screen of the receiver. In addition, the ability of the television receiver to use the color reference burst associated with the horizontal synchronization signals is degraded thereby causing inaccurate color reproduction.
Exemplary prior art sync supression systems are disclosed in U.S. Pat. Nos. 4,467,353 to Citta et al; 3,184,537 to Court et al; 3,478,166 to Reiter et al; 3,530,232 to Reiter et al and 4,222,068 to Thompson. In these prior art systems, the horizontal synchronization components of a broadcast television signal are typically suppressed or reduced to gray level and an additional keying or control signal is normally transmitted together with the television signal for controlling reconstruction or regeneration of the proper horizontal sync levels at the receiver. For example, in U.S. Pat. No. 3,184,537 an audio sub-carrier is amplitude modulated with a suitable syncinsertion control signal. In other cases, horizontal sync reconstruction at the receiver is effected by transmitting normal sync signals during the vertical interval of the television signal for enabling a timing circuit to lock to the horizontal components thereof. The timing circuit may then be used to accurately define the horizontal blanking intervals of the upcoming field to facilitate restoration of the horizontal synchronization signals in the composite baseband video signal. In U.S. Pat. No. 4,467,353, sync reconstruction is facilitated by altering the phase of all frequency components of the transmitted signal by an equal amount during its horizontal blanking intervals. The present invention is an improvement of this technique involving increased system security.